In this project we seek to elucidate the mechanisms responsible for the low CBF or ischemia found in the period immediately following severe head injury in man. We hypothesize that ischemia occurs despite adequate cerebral perfusion pressure and is not due to vasospasm of the larger, conducting vessels or too vigorous hyperventilation. As early as possible after injury, we will measure CBF, cerebral blood volume (CBV), AVDO1, AVD lactate, ICP and blood velocity with TECD, and, where clinically indicated, we will perform histological analysis of the brain microcirculation removed at surgery. Moreover, by combining the above data and data from continuous CBF (thermal dilution) and SjvO2 measurements with those from Project 1, we will support that project, and by combining CBV measurements and brain water mapping we will support Project 3.